Plant diseases in crop and ornamental plants as well as in woody plants lead annually to high economic losses. Also, in hydroponic vegetable and ornamental plant growing in greenhouses, fungi and oomycetes such as Phytophthora, Pythium and Peronospora play an important role as cause for plant diseases (Malathrakis & Goumas, 1999; Paulitz & Berlanger, 2001). In vegetable (in particular potato and tomato), fruit, ornamental plant and vine growing as well as in forestry, they are of particular economical relevance. In 2013, potatoes were globally grown at an agriculturally used area of 19.3 million hectare (Food and Agriculture Organization of the United Nations, Statistics Division). The most important pathogen in potato growing, which has gained even more importance by expanding plant growing into warmer climates, is the oomycete Phytophthora infestans, the pathogen that causes late blight (Oerke and Steiner, 1996). Its spreading is only controllable by the constant use of fungicides (more than 235 million US $ per year only for potato growing). The total market just for fungicides amounts to 5.5 billion US $ per year (Powell & Jutsum, 1993).
In Germany, approximately 100 million euros are spent annually only for the pest management in viticulture (Ochßner, 2009). In ecological viticulture, solely copper-containing plant protection products are used. However, these products are environmentally hazardous and potentially toxic. For this reason, it is of great interest to establish alternative, improved means of protection that are effective against pathogens and simultaneously ecologically friendly.
Here, the propagation cycle of oomycetes will be described using the example of Plasmopara viticola, which causes grapevine downy mildew. The life cycle is divided into two sections of varying epidemiological significance. The oospore, which is important for the survival of the pathogen during winter, is formed in the sexual phase. During the asexual summer cycle, large quantities of sporangia are released. Plasmopara viticola hibernates as oospore in the soil in leaf debris of heavily infested leafs. During late winter, the oospores become germinable and maintain their germination capacity until early summer. As soon as the soil warms up and sufficient precipitation has fallen, they germinate and form primary sporangia. Until mid-June, oospores keep germinating during heavy rains. Some oospores may also be dormant for more than a year and germinate in the following year. Usually, germination and release of zoospores originates from the primary sporangium when temperatures rise above 10° C. and more than 8 mm precipitation has fallen. Under these conditions, usually the first young leaves of grapevine are unfolded, so that the primary infection can take place. For the primary infection by the germinated zoospores, the leaves have to be sufficiently wetted with water. Solely in this phase, the infection can be prevented or reduced if damaging or inhibiting the zoospores has been successful.
The primary infection is the starting point of the summer cycle of Plasmopara viticola, in which the pathogen reproduces asexually by sporangia and may cause epidemics if the conditions for reproducing are favourable. The primary infection is followed by the incubation time, in which the pathogen matures inside the leaf without visible symptoms. A treatment of the infection is no longer possible at that time. Growth and development of the pathogen are heavily dependent on temperature, so that at higher temperatures the leaf tissue is faster penetrated by the mycelium and the oil spots appear earlier in comparison to low temperatures. At the end of the incubation time, so-called oil spots appear as a visible symptom of the fungal infection. As soon as at night the relative humidity rises above 95% and the temperatures remain above 12° C., sporangiophores protrude from the stomata of the infected lamina. The sporangia are spread by drops of water or movement of air. As soon as these drops of water contact a green part of their host plant, the zoospores hatch. Hatching of zoospores and subsequent infection occurs under optimal conditions at 24° C. within four hours. If the temperatures are lower or higher, hatching of zoospores is delayed and the process of infection is prolonged. Plasmopara viticola may infect leaves, inflorescence including stems, grapes and shoot tips if they have stomata and if they are wetted. Small drops of water are already sufficient for the infection, however, the conditions of infection are more favourable if the wetting with water is extensive and persists for a long time. After each infection, again an incubation time follows and, subsequently, sporangia will spread as soon as there is sufficient humidity at night. Plasmopara viticola belongs to the polycyclic pathogens and can undergo several propagation cycles during one growing season. If optimal conditions for the spreading of sporangia and for infections remain during longer periods and the incubation times are short due to the temperature conditions, an epidemic can develop rapidly. Drought delays the spread of Plasmopara viticola and impedes the progression of epidemics. It is possible to predict locally phases of high risk of an infection and, consequently, to take specific prophylactic preventive measures.
Under the climatic conditions prevalent in central Europe, infections by such pathogens are to be expected in every year. To what extent these infections lead to epidemics is highly dependent on the annual weather conditions, and is not predictable at the beginning of the growing period. Epidemics, for example of grapevine downy mildew (Plasmopara viticola), can become very severe in highly susceptible classical grape varieties within a few rainy days. Therefore, this infection has to be detected and controlled at an early stage. If the infestation is already in an advanced stage, a later control is no longer possible. For this reason, commercial plant growing is only possible with preventive measures against such infections. A forecasting method, which allows performing specifically preventive controls, was already developed for Plasmopara viticola at the German federal institute for viticulture (Staatliches Weinbauinstitut) and put into practice.
Currently, numerous fungicides are on offer for conventional plant growing. Solely in viticulture, 29 fungicides are approved for application in grapevine downy mildew at present.
For ecological vine growing, grapevine downy mildew is a challenge, since here a preventive treatment is indispensable and currently only copper-containing preparations (e.g. Cuprozin) are approved. Because of the known ecotoxicological concerns regarding copper, there is an urgent need to find alternatives to this agent. These alternatives, however, have to have sufficient efficacy also under high infestation rates. For years, tests have demonstrated that the vast majority of preparations that are approved as plant strengthening agents do not show satisfactory efficacy against grapevine downy mildew. Some plant strengthening agents are effective against grapevine downy mildew at low infestation rates, however, a control measure would not have been necessary here. At a higher infestation rates, which also justify combating from an commercial point of view, the efficacy of the tested preparation was insufficient. From these tests it is perceived that no biological control of grapevine downy mildew is practicable in ecological plant growing. Especially in ecological vine growing with the limited possibility to stop an epidemic, effective and practicable approaches for the biological control of epidemics are urgently needed.
Relevance, Progress and Control of Bacterial Infections
Although the number of plant-pathogenic bacteria is lower than the number of fungi-like pathogens, the damage to crop plants caused by bacterial diseases is very high. Bacteria of the genus Xanthomonas globally cause diseases in all main groups of higher plants, which are accompanied by chlorotic and necrotic lesions, wilt and rots. An example with high economic relevance is black rot in varieties of cabbage, which is caused by Xanthomonas campestris pv. campestris. Xanthomonas oryzae pv. oryzae leads to white leaves/bacterial blight of rice by infestation of rice plants, which is one of the most serious diseases in rice plants, and subsequently to major economic and social problems. Likewise, mention must be made of Pathovar X. axonopodis pv. citri, the pathogen that causes citrus cancer, and X. campestris pv. Vesicatoria, the pathogen that causes bacterial leaf spot disease on peppers and tomatoes which is of economic importance particularly in regions with a warm and humid climate. Furthermore, fire blight, caused by the pathogen Erwinia amylovora, which is subject to mandatory reporting, has to be mentioned. Host plants of E. amylovora are rosaceae such as apple, pear and quince. E. amylovora causes wilt of leaves and blossoms of infested plant, which will then turn brown or black. Moreover, the bacterial species Pseudomonas syringae, which causes various plant diseases such as cancer, wilt and spots in important crop plants such as tomato, pepper and soy bean, has to be mentioned. This widespread species is of major importance in many plants grown under glass such as tomato, cucumber and courgette.
Most of the described bacterial plant pathogens belong to the group of proteobacteria and are gram-negative organisms (e.g. Pseudomonas, Xanthomonas). However, there are also economically relevant gram-positive pathogens such as Clavibacter michiganensis ssp. Michiganensis, which causes bacterial wilt in tomatoes. This quarantine pest is of major importance in warmer and drier growing regions of tomatoes and in greenhouses.
Plant pathogenic bacteria have several strategies to survive in the environment, for example in soil, in plant material such as seeds or in insects. Insects, other animals and humans play an important role in their spreading. Water, e.g. in form of rain drops, is an important vehicle in respect of the distribution at a plant. If bacteria are transferred to a host plant, they penetrate through natural openings, such as stomata or hydathodes, through lesions of the plant. A high bacterial density as well as external conditions such as rain, high humidity or damaged spots facilitate the infection of a plant. Bacteria can easily multiply in the interior of the plant, they colonize the apoplast and damage from there the whole plant. They disturb the physiology and morphology of plants and thus, they cause disease symptoms such as necrotic spots, defoliation, scabbing, wilt or rot (De la Fuente and Burdman, 2011).
It is therefore crucial to protect crop plants against such bacterial infections and thus to secure their harvest. Various chemical compounds and copper compounds appear on the current list of plant protection agents with an antibacterial effect that are approved in Germany. Copper containing preparations are the only means which, in turn, are allowed for the use in ecological agriculture. Treatments with copper containing preparations for the control of bacteriosis have a partial effect and show their limitations as soon as the density of the bacterial inoculum passes a certain threshold. Due to the known ecotoxicological effects of copper compounds and other agrochemicals, legitimate concerns about the use of such plant protection products exist. Furthermore, even in Germany it is allowed in exceptional cases to use plant protection agents that contain antibiotics such as streptomycin to control fire blight. In other countries, streptomycin is a legitimate means against bacteriosis, but at the same time, the use of antibiotics is extremely questionable as undesirable effects on the environment as well as reductions in efficacy by development of resistance by bacteria may be encountered through undifferentiated use of antibiotics. Therefore, it is urgently needed to develop improved, highly effective and commercially relevant alternatives to these agents that are more environmentally-friendly and safer for the user.